1. Field of the Invention
The present invention relates to an amplification circuit and, more particularly, to an amplification circuit using negative feedback.
2. Related Background Art
As shown in FIG. 1, an amplification circuit using negative feedback comprises a differential amplification circuit 101 as the initial stage driven by a constant current source to stabilize a bias, a transimpedance circuit 102 for receiving an output current from the differential amplification circuit 101 as the initial stage and performing current-to-voltage conversion using a current mirror circuit, and an output buffer circuit 103 for receiving the output from the transimpedance circuit 102 and performing impedance conversion to obtain a low impedance output. This amplification circuit includes transistors Q1 to Q4 and Q11 to Q14, resistors R1, R2, and R5 to R8, a capacitor C1, and a current source I1. The circuit also includes a power supply terminal 1, a reference potential such as the GND (ground) potential 2, a buffer amplifier 3, an output terminal 4, a noninverting input terminal 5, and an inverting input terminal 6.
In the above circuit example, to perform a stable operation without any oscillation while certain negative feedback is performed from the output to the input, the frequency characteristics of the magnitude and phase of an amplifier gain in the absence of negative feedback (this gain is called an open loop gain hereinafter) must have a phase rotation amount of 180.degree. or less for the gain in the negative feedback state (0 db in FIGS. 2A and 2B), as shown in FIGS. 2A and 2B. To obtain higher stability, the phase rotation amount is preferably 120.degree. or less. For this purpose, the frequency (to be referred to as a first pole hereinafter) lower than the low-frequency gain of the open loop gain by -3 db must be lowered using the phase compensation capacitor C1. For this reason, the value of the phase compensation capacitor C1 increases. The current of the transimpedance circuit 102 for driving the phase compensation capacitor C1 is determined by the bias current value of the differential amplification circuit 101 as the initial stage. The charge/discharge rate of the phase compensation capacitor C1 lowers. In the amplification circuit using the negative feedback, stability trades off for high speed. Basically, when the stability is determined, the speed is determined accordingly. A problem may be posed depending on a circuit application condition in terms of the operation speed.